Prolactin (PRL) and placental lactogen (PL) stimulate fat deposition, weight gain, and leptin production in rodents and humans, suggesting that lactogens play roles in maternofetal lipid metabolism and obesity. The mechanisms by which lactogens promote fat storage and weight gain are unknown. We hypothesize: (a) that lactogens promote fat deposition through induction of adipogenesis, the process of differentiation of adipocytes from stromal precursors; (b) that adipogenic effects of lactogens are mediated through induction of transcription factors including PPARkhi-2; (c) that PPARkhi-2 induction is mediated through activation of Stat5; and (d) that adipogenic effects of lactogens are modulated by insulin, IGF-1, glucocorticoids, and sex steroids. To test these hypotheses we will examine effects of exogenous PRL and PL on adipogenesis in 3T3-L1 preadipocytes, primary mouse preadipocytes and embryonic fibroblasts and the effects of constitutive expression of PL in a novel 3T3-L1 cell line. To determine if lactogenic signaling is required for preadipocyte differentiation, we will compare rates of adipogenesis in preadipocytes of PRL receptor (PRLR)-deficient mice with rates of adipogenesis in cells of wild-type littermates. To characterize effects of lactogens on PPARkhi expression we will examine effects of PRL and PL on PPARkhi1 and 2 mRNA and protein levels in 3T3-L1 cells, primary preadipoyctes, mouse embryonic fibroblasts and will compare PPARkhi expression in tissues of PRLR deficient mice with that in wild-type littermates, We will examine effects of lactogens on transcriptional activation of the human and mouse PPARkhi-2 promoters expressed in 3T3-L1 cells and will compare the time course of expression of PPARkhi1I and 2 mRNAs in PRL-treated cells with that of ADD1 and c/EBPs beta, delta, and alpha. To test the hypothesis that induction of PPARkhi-2 is mediated through activation of Stat5, we will determine if lactogens induce binding of STAT 5 to consensus binding sites in the human and mPPAR c-2 promoters. We will examine the effect of mutating PPARkhi-2 Stat5 consensus sequences on lactogen-dependent activation of PPAR gamma2 transcription, and the effect of a dominant-negative STAT5 construct on the induction of PPARkhi mRNA and protein levels in 3T3-L1 cells. Finally, to test the hypothesis that the adipogenic effects of the lactogens are modulated by sex steroids, we will examine the effects of progesterone and estradiol on the adipogenic effects of PRL in 3T3-L 1 cells and the effects of progesterone supplementation on fat deposition and leptin production in PRLR-deficient mice. The results of our studies should provide new insist into the roles of pituitary and placental hormones in adipose development and function.